Lithium-ion and related batteries, collectively known as a rechargeable energy storage system (RESS), continue to be considered a clean, efficient, and environmentally responsible power source for electric vehicles and various other applications. In particular RESS technologies are being used in automotive applications as a way to supplement, in the case of hybrid electric vehicles (HEVs), or supplant, in the case of purely electric vehicles (EVs), conventional internal combustion engines (ICEs). The ability to passively store energy from stationary and portable sources, as well as from recaptured kinetic energy provided by the vehicle and its components, makes batteries ideal to serve as part of a propulsion system for cars, trucks, buses, motorcycles and related vehicular platforms. In the present context, a cell is a single electrochemical unit, whereas a battery pack is made up of one or more cells joined in series, parallel or both, depending on desired output voltage and capacity.
Battery temperature significantly affects the performance, safety, and life of lithium ion batteries in hybrid vehicles under differing driving conditions. Uneven temperature distribution in the battery pack can lead to electrically unbalanced modules, and consequently to lower performance and shorter battery life. As a result, thermal management for lithium ion batteries is receiving increased attention from automobile manufacturers and battery suppliers. Major thermal concerns of a battery pack are overheating and uneven heating within each individual battery cell and across the entire battery pack during operational charge/discharge cycles, which can lead to fast battery degradation and capacity reduction of battery cells. Maintaining a uniform temperature within the battery cell is difficult because of non-uniform heat generation within the battery cell. In addition, the heating and cooling systems can produce non-uniform heat transfer because of their internal thermal resistance. Battery pack designs in which battery cells operate in controlled temperature ranges are desirable.
The convection heat transfer rate of battery cooling plate surfaces and battery heat generation rate are the two major parameters that affect the temperature of battery cells. The generation of propulsive power from the RESS also produces significant thermal loads. As such, a RESS-based system preferably includes a cooling system to avoid unacceptably high levels of heat being imparted to the batteries and ancillary equipment. Keeping excess heat away from these, as well as other, thermally-sensitive components helps to promote their proper operation and long life. In one particular form, such a cooling system may include the passive or active circulation of a liquid coolant in, around or otherwise thermally adjacent to the batteries or other heat-generating components.
Li-ion high-performance batteries are used in hybrid powertrains exhibiting exceptionally high dynamics. At times of momentary peak load, e.g. when braking (recovery (recuperation) of brake energy) and accelerating (assisted acceleration (boosting)), batteries must generate a high output within a very short time. These momentary peak load periods generate powerful electrical currents, causing significant warming of the Li-ion cells due to internal resistance. At around 95 percent, charging and discharging efficiency is very high; however, the resulting waste heat cannot be ignored. Coupled with the fact that, in the warmer weather and in warmer climates in particular, the temperature of the vehicle interior can rise to well in excess of 40° C., operating Li-ion batteries without cooling is not an option.
A primary challenge to any battery module cooling system is to provide uniform heat-transfer from the cells so that temperature variation across the pack and within a cell is kept to a minimum. Various cooling systems dependent on circulating coolant via coolant channels in cooling plates which are in contact with the battery cells are known in the art. The current generation of cooling systems relies on single plates which circulate coolant in a U-shaped flow, having inlet and outlet ports on the same side of the plate. As the coolant traverses the plate, however, the heat transferred from the cells reduces the heat-transfer rate of the coolant and heat transfer is not uniform. Further, the friction losses in the inlet duct and outlet duct, and head loss due to flow separation at the approximately 90° cooling plate bend, cause the flow rates of coolant channels at the front end or inlet/outlet side to be higher than the flow rates of coolant channels at the back end, which results in non-uniform heat transfer rates from the front to the back.
Within individual cells unacceptably high temperatures may occur due to temperature variation based on cooling plate contact where cooling is insufficient or non-uniform. At high temperatures, the battery cells degrade more rapidly and their performance and capacity is reduced. As a result, cells may fail to achieve the prescribed ten-year life span. A possible alternative is to deactivate the batteries when temperatures exceed a set limit. All the advantages of the hybrid vehicle, however—electric boosting or recuperation of brake energy—would then be lost. For vehicles that run solely on electric power, this would even be impossible, since Li-ion batteries are the sole energy source.
Therefore, it is desirable to produce a battery module cooling system which reduces pressure drop and friction loss, and which maintains a substantially uniform coolant flow rate across the module, and which therefore reduces temperature variation across the battery pack as well as within individual battery cells.